The discovery of oxide superconductors, such as RE--Ba--Cu--O-based (wherein RE represents a rare earth element) superconductors, has enabled liquid nitrogen to be used as a cooling medium, so that application of the superconductor which was considered difficult to realize has been moved into the limelight. Typically, it has become possible for a superconducting material to be used in a bulk form without being formed into a wire rod. For example, magnetic levitation becomes possible through the utilization of repulsion against a magnet, which has led to studies on the application of the superconducting material to a bearing and a flywheel. Since the oxide superconductor is inherently a brittle material, it is difficult to produce a very long wire rod therefrom due to its brittle property. However, if it is moldable into a bulk form, the practical use of the oxide superconductor is highly promising. Namely, if a large bulk material can be formed, for example, this would enable the oxide superconductor to be applied to a ring magnet having a large bore diameter, or a magnetic shield.
The melt process represented by the MPMG (melt powdering melt growth) process (see U.S. patent application Ser. No. 08/073,656 which is a continuation of Ser. No. 07/606,207 (now abandoned)) which has been developed by the present inventors in recent years has enabled a large bulk material to be produced from a RE--Ba--CuO-based superconducting material (wherein RE represents a rare earth element including Y) while maintaining a high critical current density. The MPMG process comprises heat-melting a mixed powder comprising RE.sub.2 O.sub.3, BaCO.sub.3 and CuO in a temperature range of from 1300.degree. to 1450.degree. C., cooling the melt to give a solidified product, pulverizing the solidified product, forming the resultant powder into a predetermined shape, heating a formed body to a partially-melted state (a mixed state comprising RE.sub.2 BaCuO.sub.5 (211 phase) and a liquid phase) at a temperature of 1050.degree. to 1200.degree. C. and cooling the partially-melted formed body according to a predetermined temperature pattern to give a RE--Ba--CuO-based oxide superconductor. This enables a bulk material having a diameter of 3 cm and a height of about 2 cm to be prepared with a high reproducibility. This has led to success in levitating person through the use of such a bulk material.
When further development in the future is taken into consideration, it becomes necessary to produce a larger bulk material. Specifically, in the melt process, since the raw materials which have been melted and solidified are pulverized and then formed, in principle, any large bulk material can be produced. In an actual attempt to produce a bulk material having a diameter of 15 cm and a height of 2 cm, however, the occurrence of cracking was observed. This makes it difficult to produce a further increase in the size of the bulk material and gives rise to a problem of deterioration of properties.
It has been found that the occurrence of cracking is attributable mainly to the deformation of the formed body by dead weight during the heat treatment in a partially-molten state at 1050.degree. to 1200.degree. C. and the reaction and adhesion of the formed body to a substrate (Al.sub.2 O.sub.3, Pt or other plate) supporting the formed body which leads to the occurrence of strain in the formed body different from the coefficient of thermal expansion of the substrate in the step of cooling. In order to attain a high critical current density, it is required to conduct a heat treatment in a partially-molten state. In this case, the heat treatment should be conducted while preventing the occurrence of as much strain as possible.
In view of the above-described circumstances, an object of the present invention is to provide a process for producing an oxide superconductor which enables a large formed body of an oxide superconductor to be produced without occurrence of cracking through the elimination of as much strain as possible in the treatment of the oxide superconductor in a partially-molten state at 1050.degree. to 1200.degree. C.